"Hey have you ever seen a Lars Von Trier movie?"
"Lol who's that? He's got a weird name. My favorite movies are Despicable Me and Ted. Also, wtf happened to Shia Labeuf? He was doing cool action shit and then he, like, started failing at life and doing boring movies."
"Wow, you're such a pleb."

Short for plebian, derived from the period of the Ancient Romans. The plebeians were the general body of Roman citizens and those who were privileged were labeled patricians.
Nowadays, a pleb is one associated with those in the lower class. They often lack integrity and sophistication, while many people who consider themselves modern-day patricians often lookdown and mock them.

Man in hole: Oi. Anyone out there?
Man outside of hole: Not if there isnt anyone of my kind in there!
Man in hole: you mean someone who is outside of this hole?
Man outside of hole: chut up, pleb!

anyone that is below you. It can be socially, economically, linguistically, intelligently, whatever, it doesn't matter. It is up to you to decide and is on a case by case basis. Though if you are calling some rich bastard like Warren Buffet a pleb than your first name should probably be Sultan, to put it in perspective. It is best to use the word to describe someone that is not around though because then they are not there to prove you wrong with a fat stack of cash or by breaking out the queen's english or some crap. However, calling someone a pleb usually does not lead to a fight because they are likely too stupid to understand how much you have just insulted them. It is really best used to embrace your own current well-being, in good fun, with a underhanded insult on the dregs of society.

A guy walks out onto his pool deck in the middle of the afternoon with a tray full of double patron margaritas for his friends and lobster tails and steaks on the grill and says "I wonder what the plebs are doin today?" and then wonders if he turned on the security system at his beach house.